Light modulating apparatus and method



455-617 AU 233 EX FIPBlOb XR 293459441 March 28, 1944. w, \MLLARD2,345,441

LIGHT IODULATING APPARATUS AND IETHOD Filed Doc. 2, 1942 2 Shoots-Sheet1 M/VEN TOR GWW/LLARD A T TOR/V5 Y March 28, 1944.

' e. w. WILLARD 2,345,441

LIGHT IODULATING APPARATUS AND METHOD Filed Dec. 2, 1942 2 Sheets-Sheet?INVENTOR V G. M. WILLARD ATTORNEY Patented Mar. 28, 1944 UNITED STATES:PATENT orries nonvullzifgrrmrus AND Gerald W. Willard, Fanwood, N. 3-,asslgnor to Bell Telephone laboratories, Incorporated, New York, N. Y.Ia corporation of New York Application December 2, 1942, Serial No.467,650

16 Claims. (CL 178-1003) This invention relates to apparatus for and amethod of modulating light and particularly to an apparatus and methodmaking use of an ultra- Ionic light modulating means for producing apush-pull, variable density sound record upon a sound motion picturefilm. I

In accordance with the invention there is provided an improvedultrasonic light modulator and particularly an ultrasonic lightmodulating arrangement for producing a push-pull, variable density soundrecord upon a light sensitive film. the film being moved continuouslywith respect to the modulated light beam which is directed upon it.

There is disclosed in my Patent No. 2,287,587, granted June 23, 1942, alight valve of the ultrasonic type and it is stated therein that eitherthe undiil'racted light is intercepted and only the diifracted lighttransmitted, or, conversely, the diffracted light is intercepted andonly the undiifracted light is transmitted. In producing a push-pull,variable density sound record in accordance with the present invention,a portion of the diflracted light and a portion of the undifi'ractedlight are simultaneously transmitted by the light modulating device, oneportion of the sound track on the film being exposed to the transmitteddiflracted light and another portion of the sound track beingsimultaneously exposed to the transmitted undiil'racted light to producethe push-pull record. As the amplitude of the modulating voltageincreases, the intensity of the diflracted light reaching the filmincreases while the intensity of the undiil'racted light reaching thefilm decreases. Thus, the density of the portion of the developed filmnegative which was exposed to the diffracted light increases and thedensity of the portion of the negative which was exposed to theundiil'racted light decreases as the amplitude of the modulating voltageis in-' creased, and vice versa.

In accordance with a specific embodiment of the invention herein shownand described for the purpose of illustration, there is provided anultrasonic cell, such as is disclosed in my Patent No. 2,287,587, forexample, the piezo-electriccrystal of which is set into vibration undercontrol of a signaling electromotive force to set up compressional wavesin the transparent liquid or other wave propagating medium in the cell.Light from a source of constant intensity illuminates an entranceaperture plate having light transmitting apertures therein and the lighttransmitted through these apertures is directed through the liquid inthe ultrasonic cell in a direction substantially parallel to the wavefronts of the compressional waves setlup therein due to the vibration ofthe piezoelectric crystal. The piezoelectric crystal is set intovibration by impressing upon its electrodes an alternating electromotiveforce having a high frequency to which the crystal is resonant, thiselectromotive force being modulated with respect to amplitude by asignaling electromotive force. The portion of the cell through whichlight is directed is limited in the direction of compressional wavepropagation to a dimension which is small with respect to onehalfwave-length of the highest frequency or the modulatins e ectromotiveforce. There is also provided an exit aperture plate having an aperturetherein through which a :part of the light from the ultrasonic cellpasses to reach the motion picture film to produce a record thereon. Afirst portion of the entrance aperture plate on one side of aplane'through the ultrasonic cell parallel to the light beam andperpendicular to the wave fronts of the compressional wave in the cellhas a bar separating two light transmitting apertures, while the portionon the opposite side of the plane has a slit aperture in alignment withthe bar of the first portion. The exit aperture plate has a slitaperture upon which the bar and slit of the entrance aperture are imagedwhen there is no compressional wave in the cell so that the image of thebar is coincident with one-half of the exit slit and the image of theslit of the entrance aperture plate is coincident with the other half ofthe exit slit. An image of the exit slit of reduced size is focussedupon the film so that the length of the slit image is in a directionperpendicular to the direction of motion of the film. The total amountof light reaching the film from the ultrasonic light Valve is at alltimes substantially constant. However, as the amount of light reachingone-half of the sound track is increased due to a change in modu latingvoltage applied to the piezoelectric crystal, the amount of lightreaching the other half of the sound track is simultaneouslycorrespondingly decreased, and vice verse, to cause a pushpull, variabledensity sound record to be produced. While the entrance and exitaperture plates may be interchanged if desired, the arrangement usingthe single slit exit aperture plate has the advantage that the lightbeam transmitted through the exit aperture remains relatively narrowover its full length and the image of the exit aperture therefore mayeasily lie condensed to its proper size on the film. i I

The invention will now be described with ,tric driver, the pick-upmicrophone I reference to the which:

Fig. l is a Who view oi a sound recordernbodylns the invention; Figs. 2and 8 are diagrammatic accompanying drawings in views oi the being aplan view in section and Fig. 3 being a side elevation partly insection:

H8 4 and 5 are detailed the lines 4-4 and 4, respectively, and

m. 6 consists oi curves to which reierence will be made in explainin theoperation oi the system.

Reierring now to the drawings, there is disclosed a sound recordingsystem employing an ultrasonic light valve ior modulating light irom asource II in accordance with sound waves reaching microphone Iltoproduce a push-pull, variable density sound record upon the sound track0! a motion picture nlrn I! which is moved continuously past the area atwhich the film is to the modulated light beam. Light irom source ll ispreferably transmitted through is which transmits only a relativelynarrow portion of the light spectrum, for example, violet and nearultra-violet the dim I2 is nlost sensitive. A condensing lens is directsthe light from filter I! to illuminate the entrance aperture plate ll oiopaque material and having therein a slit aperture ll extending in onedirection irom the optical axis l1 and a bar ll separating two slits isand 2| exdirectlon irom the optical in P18. 4, the bar it the bar andslit have uminated area oi the entrance aperture screen or plate isindicated by the dash-dot line Fig. 4.

The ultrasonic cell com a metallic tank II which is completely nlledwith a suitable liquid such as xylol through a short pipe it which,after been nlled. is closed by a plug a. 23 is provided ior closing oneend oi the tank to permit a volume change oi thetankinresponsetoasmallchangeinpressure in the liquid, due to temperaturechange, ior example, as described in my application serial No. 467,651filed Dec. 2, 1942. For setting up compressional waves in the liquid oithe cell there is provided a piezoelectric driver comprising an X-cutquartz crystal 24 and electrodes 2! and 28 which are preferably iormedon the crystal sur- The electrodes rectangul r opening is provided atthe top of the tank :1 ior receiving the piezoelectric driver.

be mounted by soldering a copper ioii connecting strip 11 to the platingwhich tank 2!,

substance is for absorbing waves aiter their transmis ion through theuseiul portion oi the cell is provided, mai wool embedded in agelatinous substance. as disclosed in my application serial No. 467,652illed Dec. 2, 1942. For energizin the pieizsoelec- I conthecompressional cryst l 24 is current irom the resonant and the modmodulator ii, aiter being amlight to which for example, anising an imageoi the plifled by the amplifier 33. is impressed upon the electrodes 25,2t oi the piezoelectric driver. Spherical lenses :5 and is mounted overrectangular o enings II and ii in the opposite walls oi the tank isdirect light irom the illuminated slit apertures ii, I! and Il throughthe liquid in the cell in rays parallel or nearly parallel to the wavefronts oi the compressional waves in the liquid, and thence upon theexit aperture plate 31 having a slit aperture ll therein. As shown inFig. 5. the slit aperture at has the same dimensions as the bar is andslit It together oi the entrance aperture plate shown in Fig. 4. whenthe piezoelectric rlriver is unenergised, the lenses 8i and it iocus animage of the bar is and slit is upon the slit ll oi aperture plate 31 sothat the transmitted light spectively oi Just beyond the two sides bythe dash-dot line in Fig. 5. A lens as iocusses a reduced image of slit38 upon the sound track portion of the motion picture film l2.

As shown in Figs. 2 and 3, light irom the source Ill, after beingfiltered by the niter i3, is directed I! by a lens I4 I. Light whichpasses through the slits it, I! and ll of entrance aperture plate IIreaches the lens 85, passes through the ultrasonic cell it in rays whichare nearly arallel and is then directed by lens It to the exit apertureplate 11, the lenses :5 and a iocusslits in the entrance aperture platell upon the exit aperture plate 31. As shown in Figs. 2 and 3, when theultrasonic cell is unenergised, light coming irom the slit It inaperture plate I! (below the optical axis in Fig. 2) reaches the portionof silt ll in exit aperture plate 31. which is above the optical axis inFig. 2 and thence passes through slit :8 to reach lens a which imagesthe light slit 38 on film II. For this condition, light irom slits IIand II of entrance aperture plate I! is intercepted by the I1 andthereiore does not In some cases it may be preierred to employ anadditional lens close to the entrance aperture plate II for iocussing animage oi lens It on lens and a second additional lens near exit apertureplate I1 ior iocussing an image of lens ii on lens Is. It may also bedesirable to employ a lens system instead oi the single lens 89, asshown in the drawings. ior iocussing an image oi the exit aperture plate81 upon the illm It.

When the piezoelectric driver 24, I5, 26 is energized, the light passingthrough the liquid in the cell is diilracted in accordance with theamplitude oi the compressional wave set up in the liquid. The portion oithe cell between the piezoelectric driver 24, 25, and 2i and the waveabsorbing pad is through which light is directed is short with respectto a hall wave-length in the liquid oi the highest irequency componentoi the modulating signal generated in the circuit including themicrophone ii. This highest modulating irequency may be 10 kilocyclesand the carrier current source 32 may have a frequency of 10 magacycles,for example. with this modulating frequency, a hali wave-length inxylol, in which the wave velocity is approximately 1,300 meters persecond, is about 6.5 centimeters. Thus at any instant the amplitude oithe compressional wave in the liquid of the ultrasonic cell isrepresentative of the amplitude of the speech or music signal generatedin the circuit including the microphone H which signal is recorded "*onthe motion picture film l2.

As the amplitude of the compressional waves in the liquid is increasedfrom zero due to an increase in the alternating voltage impressed uponthe electrodes 25, 2, the light passing through the liquid in the cellis diffracted in accordance with the amplitude of the alternatingvoltage. As a result the diiiracted portion of the light from slits i8and of the entrance aperture plate passes through the portion of slit 8'in exit aperture plate 37 which is below the optical axis as viewed inFig. 2. At the same time, the difi'racted portion oi the light from slitI i of the entrance aperture plate is intercepted by the exit apertureplate 31. The total amount of light passing through the slit 38 in theexit aperture plate and reaching the film l2 remains substantiallyconstant, the amount of diffracted light passing through the portion ofthe slit 38 below the optical axis in Fig. 2 and reaching the portion ofthe film 12 above the optical axis being increased and the amount ofundiflracted light passing through the portion of slit 38 above theoptical axis and reaching the portion of film I2 below the optical axisbeing decreased simultaneously as the alternating voltage impressed uponthe electrodes of the piezoelectric driver 24, 25,

26 is increased. This relationship is graphically illustrated in Fig. 6in which curve A shows the relationship between the alternating voltageimpressed upon the piezoelectric driver and the light transmission tothe film for the portion of the light beam which is transmitted throughslit Ii and the portion of the slit aperture 3! which is above theoptical axis in Fig. 2 while curve B shows this relationship for theportion of the light beam which is transmitted through slits I! and 29and the portion of slit 38 which is below the optical axis in Fig. 2. InFig. 6, the maximum amplitude of the alternating voltage is at a valuewhich lies near the right-hand extremities of the straight portions ofthe curves lying to the left of this value. A push-pull; variabledensity sound record is thus produced upon the sound track of motionpicture film i2.

What is claimed is:

1. Apparatus for modulating light from a light source under control of asource of signaling electromotive force, comprising an ultrasonic cellin the path of a light beam from said light source having vibratingmeans for setting up mechanical waves therein under control of theelectromotive force of said source of signaling electromotive force, anentrance aperture plate in the path of the light beam entering saidultrasonic cell, an exit aperture plate in the path of the light beamleaving said ultrasonic cell, one of said aperture plates having a slitaperture therein, the other aperture plate having in one portion thereofa slit aperture and in the other portion thereof a bar which separatestwo apertures. and means in said light path .for focussing an image ofthe slit in said one aperture plate upon the bar and slit oi the otheraperture plate, or vice versa, when no signaling electromotive force isapplied to said ultrasonic cell, the bar and slit and the image iocussedthereon being substantially coextensive.

2. Light modulating apparatus comprising an ultrasonic cell in the pathof a light beam for diilracting a portion of the light beam transmittedthrough the cell under control of a compressional waveset up therein,and means in the path of the light beam for transmitting diffractedlight from said cell and for simultaneously separately transmittingundiiiracted light from said cell {Light modulating apparatus comprisinga 'transparent, compressional wave propagating medium, means fordirecting a li ht beam through said medium, means for setting up in saidmedium compressional waves for difiracting a portion of the li htpassing through said medium which varies in accordance with theamplitude of said compressional waves, and means for interceptingdifiracted light and transmitting undiffracted light of a portion of theemergent light from said medium and for intercepting the undiil'ractedlight and transmitting difiracted light or another portion of saidemergent light.

'4. Light modulating apparatus comprising a transparent, compressionalwave propagating medium, means for directing a light beam through saidmedium, means for setting up in said medium compressional waves forcausing diffraction of the light, the relative amounts of diffracted andundifiracted light emerging from the wave propagating medium varying inaccordance with the amplitude of said compressional waves. and means forderiving from said emergent light beam a light beam havingonlydifi'racted light and a light beam having only undifiracted light.

5. Apparatus for modulating a li ht beam comprising means fordiiiracting the light of said beam in accordance with and under controlof signals, and means for deriving from the emergent beam irom saidlight difiracting means two light beams. one having only light which wasdifiracted in passing through said light difiracting means and the otherhaving only light which was not diifracted in passing through said lightdim-acting means.

6. Apparatus for modulating a light beam comprising means fordifiracting the light of said beam in accordance with and under controlof signals, means for deriving from the emergent beam from said lightdiilracting means two light beams, one having only light which wasdifi'racted in passing through said light difi'racting means and theother having only light which was not .difiracted in passing throughsaid light difiracting means, and means for receiving both said lightbeams simultaneously for producing a record of said signals the totalamount of light received by said recording stantially constant and thelight of the diffracted and undinracted beams each varying in responseto signal variations.

7. The method or recording signals which comprises difiracting a portionof the light in a light beam in accordance with ignals to be recordedand utilizing diffracted and undifiracted light from said beamsimultaneously in recording said signals 8. The method of producing apush-pull signal record which comprises partially difiracting a lightbeam under the control of signals and simultaneously producing tworecords of said signals one under control of difi'racted light from saidbeam and the other under control of undiflracted light from said beam.

9. The method or recording signals by means of light which comprisesvariably difiracting a light beam in accordance with the signals to berecorded, utilizing a difiractcd portion of the light beam for producinga record or the ignals and means remaining sub- I simultaneouslyutilizing an undifiracted portion of the light beam for producin asecond record of the signals.

10. The method or recording signals by means or light which comprisesvariably difiracting a light beam in accordance with signals to berecorded, utilizing a difiracted portion or the light beam ror producinga record or the signals and simultaneously utilizing an undifiractedportion of the light beam for producing a second record of the signals.the sum of the undifiracted and diffracted portions of the light beamwhich are used for recording being substantially constant so that thesecond record is complementary with respect to the first.

11. The method of recording signals upon a light sensitive mm whichcomprises directing a substantially fixed amount oi light on the filmwhile the film is in motion, causing the light directed upon one portionof the film to be diffracted while the light directed upon anotherportion is undifiracted, and varying the ratio of difi'racted toundifiracted light in accordance with the signals to be recorded.

12. The method of recording signals upon a light sensitive film whichcomprises moving the film, difiracting light under control oi asignaling electromotive force to produce .a light beam of which theratio of diffracted to undifi'racted light varies in accordance with theamplitude variations of the signaling electromotive force, andseparately simultaneously utilizing the difiracted and the undiflractedportions of the light beam to produce two variable density records uponsaid film of which the density of one increases with increasingsignaling electromotive force and the density of the other decreaseswith increasing signaling electromotive force.

13. Apparatus for modulating light from a light source comprising afirst aperture plate for transmitting a portion of the light from saidsource, a transparent medium through which is transmitted light fromsaid first aperture plate, vibratory means for setting up compressionalwaves in said medium to cause light therein to be difiracted, the amountof light which is diliracted varying in accordance with the amplitude ofsaid compressional waves, a source of varying electromotlve iorce forcontrolling the vibration of said vibratory means, a second apertureplate for transmitting a portion of the light which is transmittedthrough said medium, a portion of the difiracted light and a portion ofthe undiliracted light being intercepted by said second aperture plate,and means including said aperture plates for causing difiracted lightonly to pass through one portion of said second aperture plate and forcausing undiliracted light only to pass through another portion of saidsecond aperture plate.

14. Apparatus in accordance with claim 13 in which the first apertureplate has a plurality of light transmitting apertures therein and thesecond aperture plate has a single light transmitting aperture therein.

15. Apparatus for producing a push-pull, variable density signal recordupon a continuously moving light sensitive film comprising an ultrasoniccell in the path of light from said source, a source of signalingelectromotive force for energizing said ultrasonic cell, an entranceaperture plate in the light path between said source and said cell, oneportion of said entrance aperture plate having two light transmittingapertures separated by a bar and another portion having a slit aperturein alignment with said bar, said slit and bar having substantially equaldimen sions, an exit aperture plate in the light path between saidultrasonic cell and the light sensitive film, said exit aperture platehaving a slit aperture, means for iocussing an image of said bar andslit of said entrance aperture plate upon the slit of said exit apertureplate when no signaling electromotive force is impressed upon saidultrasonic cell, and means for iocussing an image of the slit in saidexit aperture plate upon said light sensitive film.

l6. Ultrasonic light modulating means comprising a transparent mediumthrough which compressional waves may be propagated for causingdiffraction of light, the amount of difiracted light increasing and theamount of undifiracted light decreasing as the amplitude of saidcompressional waves is increased, and vice versa, light sensitive means,and means for directing a light beam through said medium to said lightsensitive means and for intercepting a portion of the difiracted lightand a portion of the undifiracted light so that one portion of saidlight sensitive mean; is illuminated only by difiraeted light whileanother portion is simultaneously illuminated only by undifiractedlight.

GERALD W. WILLARD.

